My contest to try and get some real figures on the round-trip efficiency of pumped hydro was a dud! No studies, no real world numbers. I heard from a few helpful readers that had projected efficiency numbers based on new turbines from Japan, but no information on existing projects. I find this highly interesting because new advanced energy storage systems, like the VRB-ESS(tm), are being compared, sometimes unfavorably, against pumped hydro. If PH is the gold standard for highly efficient time-shifting power generation, then one would think there would be substantial and easily accessed information on real-world experience.
I may try this again. In the meantime, if any reader has any resources on the subject, please leave a comment.
While we're on the subject, here are a couple of interesting points on the electricity in - electricity out efficiency of the VRB. The standard metric is 65% - 75%, AC-AC. However, the actual round-trip efficiency depends on the application.
For example, the lithium battery providers, like A123 and Altairnano, and flywheel providers, like Beacon Power, are advertising 90% efficiency. However, this is for a very limited pulse of power in the middle of their state of charge (SOS). The application is used for balancing the 60Hz frequency of the grid by pulsing to full capacity - in megawatt size - for only 15 minutes. Apparently, just about any battery system, including lead acid, could pulse like this, in the middle of their SOC, with high efficiency and many cycles.
This applies to the VRB-ESS as well. The greatest efficiency loss occurs at the end of the charge cycle, as it takes more work to find "uncharged" vanadium ions to "fill up the tank". In other words, if a facility had installed enough tanks of electrolyte to store 8 hours of energy, but only used the first 6 hours, then the round-trip efficiency would be closer to 80% than 70%, and if the VRB-ESS was used for the same application as the lithium systems, the efficiency would be the same - in the 90% range. However, one of the key distinctives of the VRB is the ability to fully cycle a nearly unlimited number of times without loss of capacity. Capacity is dependent on the amount of electrolyte. So, an application could chose 30 minutes of storage or 8 hours; it's simply a function of how much electrolyte is stored in the tank. After thousands of full cycles, whether on a 30 minute tank of electrolyte, or an 8 hour tank, the VRB is still able to provide full capacity and for the same amount of energy.